The present invention relates to a semiconductor laser element, and more particularly to a stripe-geometry double heterojunction (DH) laser element which has higher order transverse mode oscillations suppressed and provides an extremely high optical output.
Heretofore, various types of double heterojunction laser elements represented by double heterojunctions of (Al.Ga)As series crystals have received attention as a light source for use in optical fiber communications because they readily produce continuous wave (CW) oscillations at room temperature. As is now well known in the art, a double heterostructure portion typically comprises an n-GaAs substrate on which are grown the following expitaxial layers in the order recited: an n-Al.sub.x Ga.sub.1-x As layer, x&gt;0; an n- or p- or compensated Al.sub.y Ga.sub.1-y As layer, 0.ltoreq.y&lt;x; and a p-Al.sub.z Ga.sub.1-z As layer, z&lt;y. However, in a stripe-geometry laser having its electrode formed in a stripe shape, especially in an (Al.Ga)As laser having a stripe electrode width of 10.about.20 .mu.m, a CW optical output of 10 mW and a pulse-operated (pulse width of 100 ns) optical output of 100 mW were the threshold of operations, and if an optical output beyond this threshold is emitted, then one of the reflective surfaces may be easily destroyed. This phenomenon has been known as catastrophic optical damage (COD), and the threshold optical output power density is about 1 MW/cm.sup.2. In the prior art, various trials for lowering an optical power output density on a reflective surface have been made for attaining a high optical output. More particularly, enlargement of a stripe electrode width, increase of an active layer thickness, double-double heterostructure (DDH), etc. have been reported. However, in these cases, there was always an associated increase of a threshold current density which made continuous wave oscillation at room temperature difficult. In addition, if the laser is made to operate at a high optical output, then an oscillation region is expanded even though the stripe electrode width is narrow, so that a horizontal transverse mode (a transverse mode in the direction parallel to an active layer) becomes complex multiple modes. Consequently, use of high optical output semiconductor lasers has been limited to detection of obstacles or the like, and new uses such as high optical output light communications, laser printers, etc. have been not realized.
The cause of generation of catastrophic optical damage is conceived as follows: In the conventional semiconductor lasers, an active layer is fully injected and activated up to the neighborhood of reflective surfaces. However, in the close vicinity of each reflective surface, an injected carrier density is lowered as influenced by the surface states, so that the vicinity acts as an absorber for the emitted light. If the optical output is increased, light absorption in the vicinity of each reflective surface increases, resulting in an increase of heat generation or Brillouin scattering, and thus destruction occurs. The reason why the threshold output power densities in the heretofore known lasers in which a high optical output was aimed at as by varying an active layer thickness or by providing a p-n homojunction within a thick active layer, were all about 1 MW/cm.sup.2 (pulsed operation), exists in the above-mentioned basis.
In addition, a laser structure for enhancing the threshold optical output for catastrophic optical damage by making the vicinity of one reflective surface as a low loss region for laser light, that is, by not exposing one end of an active layer on the reflective surface, was proposed by B. W. Hakki in U.S. Pat. No. 3,824,493 (July 16, 1974). However, since this structure has a low loss region provided only in the vicinity of one reflective surface, upon high optical output operation the other reflective surface would be subjected to catastrophic optical damage. Furthermore, since an exciting region in an active layer is defined only by a stripe electrode, there is a shortcoming that horizontal transverse mode oscillations would be easily turned to higher order multiple modes.